A review of the prior art which is pertinent to the present invention considers two general areas. The first of these is that prior art which deals with the manufacture of rigid-flex circuit boards. This is followed by a review of the prior art relating to prepreg materials used in the manufacture of laminated structural components.
Techniques for making multilayer rigid flex printed circuit boards are well known in the field. One early example of the prior art is disclosed in U.S. Pat. No. 3,409,732 assigned to the assignee of the present application. Typically, a rigid flex stacked printed circuit board includes flexible printed circuit cables extending from the periphery of the rigid section or sections. The rigid portions of the flex cables are typically used as sites for electronic components or mechanical hardware. It is important to note that the copper conductor in each plane or layer is fabricated from one continuous sheet of copper foil.
Typically, and as disclosed in detail in U.S. Pat. No. 4,800,461, also assigned to the assignee of the present invention, in the construction of a multilayer rigid flex circuit board, the initial processing step includes formation of a basestock by laminating two copper sheets to an insulator layer comprising one or two fiberglass sheets impregnated with an adhesive such as an epoxy, commonly referred to as a prepreg. Following lamination, the copper layers can be imaged and etched to provide copper pads and conductors. The exposed copper conductor patterns are then treated to enhance bondability of an epoxy prepreg to the copper. Then, two additional insulator prepreg sheets having cutouts are positioned on both sides of the base stock. A flexible insulator of Kapton (polyimide) covered with a suitable adhesive which provides bonding to copper is positioned on both sides of the cutout section. In addition, the Kapton sheets are slightly longer than the cutout sections in the glass layers to overlap by, for example. 0.050 inch. The sandwich formed by the foregoing sheets is then laminated together to provide a rigid flex board, wherein the Kapton provides excellent flexibility and tear resistance characteristics to the flex section.
Of additional general background interest, attention is directed to U.S. Pat. No. 5,499,444, entitled "Method of Manufacturing a Rigid Flex Printed Circuit Board". In said patent, it is reported the fabrication of multilayer boards with the above referenced materials has led to some persistent problems. First, alignment of circuitry at different layers in the board is critical, and provisions must be made to prevent sliding of any layers in different planes with respect to other planes by more than a tolerance of a few thousandths of an inch. Maintaining registration of the flexible portion has also been a serious problem since the hard board must be cured or laminated by a heat-press process that is likely to cause interlayer slippage as well as thermal dimensional changes. Other problems are encountered due to the thermal expansion of the typically used insulator materials such as acrylic adhesive and the polyimide film utilized in the construction ol the rigid flex boards. Thus, failures occur when the board is subjected to elevated temperatures in thermal stress testing, hot oil solder reflow, and the like.
Another difficulty with the use of dielectric films such as Kapton film in the rigid board area is their absorption of excessive moisture, on the order of up to 3 percent by weight of water. Absorbed moisture in the circuitry with no means of escape, may cause unacceptable delamination in the rigid board area when it volatilizes during fabrication or during subsequent high temperature operation. This effect may be more destructive than simple mismatch of thermal coefficients.
The foregoing problems have in one form or another been addressed in the prior art, as shown for example in U.S. Pat. No. 4,800,641; U.S. Pat. No. 5,144,742 and U.S. Pat. No. 5,004,639. Approaches in addressing one or more of the problems involve such construction techniques as adding pads in non functional layers of the plated through holes, utilizing a curable liquid dielectric for certain layers orf layers; using temporary sheet patches of filler material surrounding(y flex regions during a heat pressing assembly stage to maintain alignment; adding glass fiber reinforcement of the flex layer for strengthening; and finally, a number of other changes or addition to the manufacturing process. However each of these solutions entails additional steps to address any one problem, and may have adverse consequences on another apparently unrelated problem. For example, the switch to acrylic cements and a polyimide flexible film has been largely responsible for moisture absorption and failure of plated through holes in multilayer boards at high temperatures. Reducing the size of the polyimide/acrylic components to limit thermal stress introduces boundary problems where flexible and rigid elements are patched together. Furthermore many of these improvements require more detailed manufacturing steps that can be costly as well as time consuming.
In U.S. Pat. No. 5,499,444, a rigid flex printed circuit board is prepared via a process wherein each circuit layer is punched in the peripheral regions with alignment slots and all layers are assembled without any acrylic bonding in a single hot press operation. More specifically, a central layer is formed of a sheet of epoxy/glass material with top and bottom copper foil layers. This central layer is cured and punched with slots at its edge, of which a portion thereof ultimately provides formation of the flexible portion of the finished board. The slots allow motion along only one of two orthogonal axis, and subsequently the punched layers (with windows for formation of the flexible region) and the central portion are assembled in a single press curing operation in which process-induced motions and realignments are constrained to occur with a small magnitude that varies with the radial position along an alignment rosette centered on the board. A top or final cover layer extends over both the flexible and rigid regions to further assure uniform alignment at all levels during pressure assembly. The '444 patent emphasizes that by using a glass/epoxy layer for the central flexible portion instead of some form of polyimide or other material, an all glass construction is achieved that is free of the major problem of z-axis thermal stresses of the conventional polyimide-glass construction. Furthermore, it is mentioned that on top of the copper layer in the flex section is a cover of insulating material which may be an adhesively bonded plastic film or a coated-on film, such as a conformal cover coat known in the art, suitable of which is a solder mask such as a UV curable flexible solder resist or a heat curable preparation both of which can be applied by a screening operation. This cover material is then described as being cured before assembly with subsequent layers.
In U.S. Pat. No. 5,144,742 there is disclosed a rigid flex printed circuit board fabricated by the steps of first forming circuitry components on a rigid flex subassembly including laminated conductive layers on opposite sides of a central insulating layer. This is followed by depositing a liquid precursor of flexible insulating layers over the circuitry components in portions of the printed circuit board subassembly corresponding to the flexible section in the final rigid flex board. The liquid precursor is then cured to form an insulating layer as a protective coating over the circuitry components in the flexible section, which is followed by finally laminating a plurality of components including at least one rigid flex subassembly and rigidizing insulating layers to form a rigid flex printed circuit board.
Attention is next directed to "High Resolution Photoimageable Covercoats for Flex Applications", a paper presented at "Flexcon 95" by W. J. Thatcher and P. M. Banks. As disclosed therein, the product sold under the tradename Imageflex.TM. has become available, and is characterized as a thermal hardening two/pack two component liquid photoimageable flexible soldermask that dries by evaporation to give an aqueous processable film with a gloss or matt finish. It is mentioned therein that Imageflex.TM. has proven suitable in a variety of flexible circuit application. The Imageflex.TM. is said to offer significant advantages for the product of many types of flexible printed circuits, and as an alternative to polyimide coverlay, Imageflex.TM. offers lower cost and higher feature resolution and alignment accuracy. In addition, a photoimageable solder mask is now available from Taiyo, Japan.
In U.S. patent application Ser. No. 08/800,844 filed Feb. 14, 1997, entitled "Multilayer Combined Rigid/Flex Printed Circuit Board", and which is assigned to the present assignee, whose teachings and prior art cited therein are incorporated by reference, there is disclosed a multilayer rigid flex printed circuit board wherein the board laminate comprises a basestock composite containing a flexible core, formed by laminating a first conductive layer to a flexible insulator layer, along with a second insulator layer affixed to the basestock, said second insulator layer having a cutout region proximate to the flexible core of the basestock composite to expose a portion of said first conducting layer on said flexible core, and a second conductive layer also having a cutout region proximate to the flexible core of the basestock composite. A photo-imageable soldermask is then applied to said exposed portion of said first conductive layer, and to the second conductive layer, wherein said photoimageable soldermask allows for photo definition of openings upon the conductive layers upon which it is applied.
Finally, attention is also directed to U.S. application Ser. No. 08/702,729, filed Sep. 6, 1996, also assigned to the present assignee, whose teachings and prior art cited therein are incorporated by references which discloses a multilayer rigid flex printed circuit board, wherein the board laminate comprises a double-sided basestock composite, formed by laminating two conducting sheets to an insulating layer, said insulator layer containing a flexible core, a second insulator layer affixed to each side of the basestock. said insulator having a cutout region proximate to the flexible core of the basestock composite, a flexible layer affixed to said cutout regions with an adhesive, wherein said flexible layer contacts the conducting layers and abuts and overlaps a portion of the second insulator layer such that upon stacking of the board laminate a hollow region is produced as between the stacked laminate sections.
Accordingly, as can be seen from the above review of the prior art, in the case of manufacturing, a rigid flex printed circuit board, there has been an on-going effort to develop the most efficient and cost effective manufacturing process to address the various problems in the art, and in particular, the problem of combining dissimilar materials to provide rigid-flex characteristics. In other words, until now, prepreg material has been used almost exclusively for the manufacture of the rigid section, and something other than prepreg, e.g., a polyimide, has been employed for manufacture of the flex section. and as a consequence, a truly low cost rigid-flex circuit has not yet been available to the industry.
Of course, with regards to prepreg material itself, a fair amount of art has been established regarding the use of prepreg material for basic structural applications, well outside the scope of rigid flex circuitry. Nevertheless, and as noted at the outset, a review of such technology is still in order.
For example, in U.S. Pat. No. 4,927,581, there is disclosed a method of shaping elongated composite structures of a resin matrix reinforced with randomly broken fibers oriented in transverse and non-transverse directions with respect to the longitudinal axis of the structure. The formed structure is said to be characterized by the particular orientation of the fibers in the transverse and non-transverse direction after the structure is formed.
In U.S. Pat. No. 5,017,312 there is disclosed an apparatus for the manufacture of oriented chopped glass fiber mats from non-conductive fiber feed stock. A preferred embodiment includes the incorporation of a glass fiber mat into a composite by applying a matrix resin to the mat in sufficient quantity to ensure the integrity of the mats. The process for manufacturing non-woven oriented mats is also disclosed.
In U.S. Pat. No. 5,200,246 there is disclosed a low-cost, high web integrity fabric that can be economically produced and tailored to provide a variety of different combinations of characteristics and properties for different end uses. The fabric is described as containing continuous filaments, ranging from elastomeric to non-elastic. but elongatable to some minimum extent, for strength and elasticity.
In U.S. Pat. No. 5,273,819 there is disclosed an improved ram extruded composite product. A composite is said to be formed from a phenolic resin, containing carbon fibers of various lengths where each length of fibers are oriented in a pre-selected, pre-determined orientation.
In U.S. Pat. No. 5,275,877 there is disclosed a shaped reinforced thermoplastic composite. The composite is formed by forming a prepreg as a plurality of individual sheets or layers which comprises essentially unidirectionally oriented fibers. A lay-up is formed by curing each individual sheet or layer into pieces so that the direction of fiber orientation in each such piece is either parallel to one pair of edges or at angles of 45 degrees to all of the edges.
It is therefore an object of the present invention to advance further over the prior art reviewed above and provide what is considered to be one of the most cost effective routes for the preparation of a rigid flex circuit board containing prepreg, wherein the rigid flex circuit board is of homogenous construction, i.e. the prepreg therein serves as both the rigid and flex section thereof and wherein the prepreg operates in such novel manner as a consequence of selected prepreg fiber positioning and/or orientation with respect to the conductor patterns or pathways designed thereupon.
More specifically, it is an object of this invention to utilize either the positioning and/or orientation of the fibers in the prepreg with respect to the conductor patterns or pathways or to position the conductor patterns and pathways with respect to the fiber orientation in a manner which uniquely provides for the manufacture a rigid flex circuit board, wherein the flexibility of the flex section can itself be adjusted to a desired or targeted level are a consequence of such novel and alternative positioning techniques.